1. Field of the Invention
The present invention lies in the art of glass manufacture. More particularly, the invention concerns the processing of cullet which is fed to a glass melting furnace along with other raw materials. In particular, the invention is directed to a method and apparatus for preparing cullet for use in an electrostatic bed filter to remove pollutants from the melting furnace exhaust gases.
2. Description of the Prior Art
Glass manufacture involves the mixing of various batch ingredients, generally including silica sand, dry powders, granular oxides, carbonates, and other raw materials (depending on the desired glass type) and heating them to a temperature of about 1500.degree. C. where they become molten and acquire a homogeneous nature. Substantial quantities of heat are required for the melting process, generally supplied by combustion of fossil fuels. Because of the relatively poor heat transfer from the hot combustion gases to the pool of molten glass, exhaust gas temperatures from the process are usually quite high in spite of various types of heat recovery equipment employed. As well, pollutants of various types are emitted from the melting process along with the exhaust gases.
Cullet, that is, broken pieces of glass, is added to the other batch ingredients and charged to the melting furnace. A certain minimum proportion of the total batch is required to be cullet in order to provide proper melting characteristics, generally in the range of 10-20% by weight. Cullet normally used for this purpose is generated within the glass factory, either from product breakage during the manufacturing process, or from dumping of molten glass during product changes.
Recent emphasis on waste recycling has resulted in the collection of large quantities of what is called ecological cullet. This is generally glass bottles returned to recycling centers. With proper processing such as sorting by color, removing foreign substances and crushing to smaller sized pieces, ecological cullet can be made suitable for remelting into new glass. Currently there are a number of glass factories set up with glass melting furnaces where some 80-90% of the batch feed material is ecological cullet.
When used in these quantities, the possibility of preheating cullet with waste exhaust gases from the furnace becomes economically attractive and equipment for such purpose is commercially available. See, Zippe, "Economics of Cullet Preheating", Glass International, June 1992.
One such type of equipment potentially useful in this role is an electrostatic granular bed filter (EGB filter). EGB filters employ a bed of electrically charged granules. Exhaust gases containing particulate pollution are electrostatically charged and contacted with the granular bed. The electrostatically charged particles in the exhaust gases are attracted to and retained on the electrically charged granular bed.
U.S. Pat. No. 4,338,113, incorporated herein by reference, discloses methods for utilizing EGB filter technology for air pollution reduction from glass furnaces. However, this patent provides no teaching of a workable system for replacing the granules in the EGB filter with cullet.
The simple substitution of cullet for the widely used silica-based granules in an EGB filter results in several operational problems which render the system ineffective and inoperative. First, cullet is generally wet either from washing in the waste recycling process or from storage in open outside locations. Wet granules in an EGB filter would present an electrical short circuit and prevent the necessary application of high voltage to the bed.
Second, the heat transfer resulting from direct contact of the hot exhaust gases and the cullet is quite good. As a result, the exhaust gases are very effectively cooled by the cold cullet as it is first contacted by the gases. Continued heat transfer cools the gases to below their acid dew point temperature, at which point an acid mist would be formed which would in turn result in excessive equipment corrosion.
Third, as cullet is handled mechanically, the glass is subject to breakage and formation of substantial quantities of fine glass dust. When exhaust gases are passed through the cullet in the EGB filter, this glass dust would become entrained in the gases and exit the filter as dust emissions.
Hence, there remains a need in the art for a method and apparatus for preheating cullet and simultaneously using the cullet to remove pollutants from melting furnace exhaust